A multiple input multiple output (MIMO) technology that uses plural antennas for transmission and reception and transmits plural data streams in the same frequency band has been employed in cellular systems, wireless LAN systems, and so forth. Here, as the cellular system, Long Term Evolution (LTE), LTE-Advanced (LTE-A), and so forth, which are standardized in the Third Generation Partnership Project (3GPP), have been known. As the wireless LAN system, IEEE 802.11ac and so forth, which are standardized in the Institute of Electrical and ElectroNics Engineers, Inc. (IEEE) have been known.
Among MIMO technologies, the MIMO transmission in which one terminal device that has plural receive antennas communicates with a base station device that has plural transmit antennas is referred to as single user MIMO (SU-MIMO). In the SU-MIMO, the terminal device demultiplexes plural multiplexed streams by MIMO signal demultiplexing based on channel state information (CSI) with respect to radio signals which are transmitted from the base station device and in which plural data streams are spatially multiplexed.
In a case where the base station device perceives the CSI as known, the base station device forms a virtual orthogonal channel by performing precoding and thereby performs transmission. This transmission is referred to as eigenmode transmission. The eigenmode transmission is also referred to as singular value decomposition-multiple input multiple output (SVD-MIMO) or eigenbeam-spatial division multiplex (E-SDM). In the eigenmode transmission, the base station device performs singular value decomposition (SVD) or the precoding based on singular value decomposition for a transmission data sequence. The precoding in the eigenmode transmission enables the terminal device to easily demultiplex the multiplexed streams. Further, the base station device performs adaptive power allocation, adaptive modulation, or the like with respect to the virtual orthogonal channel and may thereby improve spectral efficiency.
Further, the MIMO transmission that performs communication while assuming plural terminal devices as virtual large-scale array antennas is referred to as multi-user MIMO (MU-MIMO). The MU-MIMO is effective for an improvement in the spectral efficiency and is employed in the LTE and so forth.
In the MU-MIMO, the base station device is simultaneously connected with plural terminal devices and performs communication. However, in the MU-MIMO, a signal addressed to a certain terminal device is received as an interference signal by the other terminal devices (multi-user interference). Accordingly, in the MU-MIMO, the base station device performs the precoding for the transmission data sequence and thereby suppresses the multi-user interference that is received by the terminal devices.
In the MU-MIMO, a closed-loop type MU-MIMO has been suggested. The closed-loop type MU-MIMO is MIMO transmission that uses the SVD or the precoding based on the singular value decomposition. In the closed-loop type MU-MIMO, similarly to the eigenmode transmission, each of the terminal devices feeds back information for forming the virtual orthogonal channel to the base station device. The base station device performs a linear or non-linear precoding process for the information that is fed back and thereby cancels the interference among the terminal devices.
In the SU-MIMO and the MU-MIMO, the base station device has to recognize the channel between the base station device and the terminal device in order to perform the precoding. However, in a case of a wireless communication system based on frequency division duplex (FDD) that uses different carrier frequencies for up and down links, the channels for the up and down links are different. In this case, the terminal device estimates channel state information and notifies (feeds back to) the base station device of the channel state information by using an uplink resource. Accordingly, the base station device may recognize the channel state information to the terminal device. Taking into account the change in time in the channel state information and a large increase in uplink overhead, the information of which the terminal device notifies the base station device (feedback information) is preferably compressed to an appropriate amount.
A compression method in a multi-carrier system by time-frequency transform has been suggested (NPL 1). In NPL 1, the terminal device performs inverse discrete Fourier transform (IDFT) for the channel state information that is expressed in a frequency domain and thereby acquires the channel state information that is expressed in a time domain. The terminal device only extracts components with high power of the channel state information that is expressed in the time domain.
Further, in a case of taking into account the eigenmode transmission and the closed-loop type MU-MIMO, the terminal device does not feed back estimated channel state information but may notify the base station device of the information of a right singular vector. The right singular vector may be obtained by performing the SVD, the singular value decomposition, or the like for the channel state information. In addition, PTL 1 discloses a method in which the terminal device performs phase rotation for the right singular vector and thereby compresses an information amount to be notified to the base station device. As described above, the terminal device may compress the feedback information itself by using properties of the right singular vector. Further, the terminal device adaptively changes the number of eigenvectors to feed back to the base station device in accordance with the number of transmission streams and may thereby adjust a feedback amount.